This invention relates to novel reactive compositions of an elastomer dissolved in a vinyl monomer and graft copolymers prepared therefrom that are especially tough elastomers.
The art is replete with compositions comprising mixtures of plastic resins and elastomeric materials. Many commonly used plastics such as poly(styrene), poly(methacrylate), the nylon resins, and so on, are subject to brittle fracture in their unmodified form, and it has become common to compound such resins with various proportions of rubber-like polymers to improve their resistance to breaking as the result of sharply applied stress. The sophistication of this art has also extended to the employment of elastomeric polymers that form ionic, covalent, or Van der Wals bonds with the host resin, it having been found that improvement of impact strength is even greater if some bonding occurs between the resin and the elastomeric toughening agent. Among the many patents describing such compositions, U.S. Pat. Nos. 3,489,822, 3,819,765 and 3,886,233 can be mentioned as representative. These patents describe compositions wherein minor amounts of ethylene-propylene (EP) copolymers or ethylene-propylene-diene (EPDM) elastomers are reacted with polymerizable monomers, such as styrene and acrylonitrile, to form grafted copolymers.
Elastomers are vulcanized with a great variety of chemicals, depending on the composition of the host polymer. Sulfur and its compounds, various organic compounds, metal oxides and hydroxides, and a variety of organic peroxides are commonly used. U.S. Pat. No. 3,179,715 teaches that EP copolymers can be vulcanized by a co-curing reaction in mixtures containing about 5% to 29% styrene or divinylbenzene under the influence of organic peroxide free radical initiators to give a fairly tough rubber.
The compounding and vulcanization of elastomers in conventional commercial practice is a complex process, involving mixing of polymer, fillers and curing agents in large and expensive equipment, processing on sheet-off mills, preparation of preformed portions of the resulting stock, and finally vulcanization at high temperatures in compression molding equipment. There is a great desire in the rubber industry to make greater use of the less complex injection molding methods normally used for fabricating objects of the thermoplastic resins by the plastics industry. Progress has been made in this direction by employing reactive materials such as the liquid isocyanate elastomers, which are mixed with curing and extending compounds in the course of injection molding to form polyurethane products. This process is known as liquid injection molding (LIM) or liquid reaction molding (LRM) and is described in, among other places, "Rubber Age", July 1975, pages 46-48. This type of system is used for forming polyurethane parts, for instance, and requires precise control of the metering of two or more reaction streams because proper stoichiometry of the reactants must be maintained. An analogous process using highly reactive but more conventional rubber compositions such as specially compounded SBR or EPDM rubbers of a less fluid nature is known as reaction injection molding (RIM). This process requires especially heavy duty processing equipment because of the high viscosity of the high molecular weight elastomers which are the major components of the compositions employed. One apparatus suitable for processing such compositions is described in U.S. Pat. No. 3,878,285 to Souffie. There is a need in the industry for less complex, less costly, and more easily processable compositions that can be used in the LIM and thermoset injection molding processes to give tough, high quality reinforced rubber objects.